Various, in some cases standardized methods for testing the scratch resistance of surfaces are known from the prior art. All these processes have the common feature that the surfaces of test specimens are scratched by a contacting relative movement by solids of high hardness either with several contact points, for example by loose or bound particles, or with only one contact point, for example a diamond tip. With all the methods described, as a rule the optical and/or topographical properties of the surface are analyzed after the scratching of the surfaces. This is carried out e.g. by measurement of the optical haze or of the gloss or by examinations under a light, scanning electron or atomic microscope.
The standardized methods for testing the scratch resistance include the sand trickling method (DIN 52 348). In the sand trickling method, the surface of a test specimen is scratched by a well-defined standard sand which falls through a fall pipe from a height of 1,650 mm. The amount of sand is specified here as 3 kg. The impact speed of the sand results directly from the height of fall (ignoring air friction) as 5.69 m/s. However, in respect in particular of simulation of the exposure of the surfaces of vehicle components to abrasion in the relative wind due to drift sand, dirt particles or the like, the impact speed of the sand trickling method is too low. The impact speed of particles in the relative wind is usually between about 30 km/h and 200 km/h, i.e. between 8.33 m/s and 55.56 m/s.
Another standardized test method is the abrasive disc method, also called the Taber Abraser test (DIN 52 347). In the Taber Abraser test, the surfaces of the test specimens, which lie on the rotary plate of the abrasion tester, are exposed to sliding wear by two abrasive discs rotating in the opposite direction. The abrasive discs of Teledyne Taber (USA), type CS 10 F are made of a defined fine-grained abrasive embedded in rubber. For simulation of the exposure of the surfaces of vehicle components to abrasion in the relative wind by drift sand, dirt particles or the like, the Taber Abraser test has the disadvantage that the contact force of the abrading medium on the test specimens, either 2.7 N or 5.4 N, is too high compared with the range relevant to automobile applications. Model estimations of the contact force of particles in the relative wind give values of about 0.5 N. Furthermore, this contact force occurs only over a period of less than 1 μs.
E. W. J. Mardles, J. Oil Colour Chem. Assocn. (1928), 11, pages 230-259 and P. H. Shipway and I. M. Hutchings, Surface and Coatings Technology (1995), 71(1), pages 1-8 describe methods in which abrasive particles are blasted on to a specimen surface by a stream of air. With these methods comparatively high relative speeds between the abrading medium and specimen surface of up to 77 m/s indeed arise. However, a disadvantage of these methods is that the angle of incident flow cannot be varied. In the methods described in the standards ASTM G 76-95 and ÖNORM M 8126, a stream of particles is likewise directed on to a surface at a high speed. However, all these methods have the common feature that the exit of the nozzle tube or the like which guides the gas particle stream to the specimen surface is at a distance from the test specimen, which is held freely in space. This means that the gas particle stream flows freely between the exit of the nozzle tube and the specimen surface, which can lead to swirling and turbulence in the region of the specimen surface. A well-defined flow and therefore a reproducible exposure of the specimen surface to scratching thus does not exist.
The object of the present invention was to provide a device for testing the scratch resistance of surfaces which does not have the disadvantages mentioned. The object is achieved according to the invention by the features of claim 1.